The present invention relates to a control device for a loading and unloading mechanism, and more particularly to a control device for a loading and unloading mechanism incorporated in a fork lift truck and effecting a running attitude control due to lifting height control of a fork or tilting angle control of an upright. Particularly, the present invention is concerned with a control device for a loading and unloading mechanism for effecting an operation for horizontally positioning a fork or running attitude operation in relation to a lifting height control. Specifically, the present invention relates to a control device for a loading and unloading mechanism automatically controlled in accordance with lifting height data and/or tilting angle data stored in a microcomputer.
As is well known, a fork lift truck comprises a loading and unloading mechanism and a vehicle body. The loading and unloading mechanism comprises a vertically elongated guide rail called an "upright", and a fork slidable in the upright. The mechanism further comprises a hydraulic member, as for example, a hydraulic cylinder for lifting and lowering the fork and tilting the upright.
In connection with the prior art loading and unloading control, for instance, lifting height control, drawbacks are pointed out as follows: Recently, there is a tendency that the lifting height becomes high when loading and unloading work is effected with a fork lift truck. For instance, the piling and unloading may be effected at heights greater than 10 m. In such a case, it is difficult for an operator to adjust the loading and unloading mechanism so that the fork is placed at the predetermined height, looking at the top of the fork positioned above about 10 m relatively to the seat of the operator. Accordingly, it is desirable for the operator to easily effect piling and unloading the load at the predetermined position.
In order to embody this requirement in the prior art, the upright is provided with a limit switch for stopping the fork at a predetermined position. When the fork reaches the predetermined position, for instance, 8.5 m, the control device is designed so as to light a lamp provided at the operator's unit or break a driving power supply for loading and unloading operation. Usually, a load is unloaded on a shelf with a plurality of steps. For this reason, in order to determine the desired position it is required to select the step. The provision of a predetermined number of limit switches, for instance ten, is required in order to meet the height of the shelf. Further, it happens that the piling and unloading is required at the another shelf according to the change of the working place. In such a case, if the height of the shelf is different from that of the prior one, a more complicated control device is required. Actually, it has been impossible to effect the piling and unloading operation.
Reference is made to a method for performing a loading and unloading operation. The method comprises the steps of running a fork lift truck to the position for piling a load, lifting a fork to the lifting height position, advancing the fork lift truck, mounting a load on a fork, adjusting a tilting angle of an upright in order to horizontally position the fork, and lowering the fork to the position required for safe running. The method further comprises the steps of tilting the upright in the backward direction by an angle suitable for safe running, running the fork lift truck to the position for unloading a load, and tilting the upright in the forward direction in order to horizontally position the fork after the fork is lifted to the position required for unloading, or effect the lifting height operation of the fork and the tilting operation in the forward direction at the same time. Thereafter, the unloading operation follows in a reverse order. For a second time, the reverse operation is effected so that the fork is placed in the running attitude. The fork lift truck is returned to the position for piling.
As stated above, the prior art loading and unloading operation effected with a fork lift truck requires an operation for lifting and lowering a fork, an operation for tilting an upright, and a running operation in accordance with a complicated procedure with respect to each loading and unloading operation, with the result that the efficiency of the work is lowered. Further, as stated above, when a load is unloaded, the lifting height operation of the fork and the tilting angle operation of the upright are carried out at the same time or the tilting angle operation is effected and thereafter the backwardly inclining operation is effected. Accordingly, the lifting height operation is effected under the condition that the load is not placed in perfect horizontal condition, thereby to become unstable, which brings about a safety problem.
Further, from the point of view of the system control in the prior art, a plurality of analog control circuits, such as, comprising combination of relay circuits respectively provided with respect to the controlled system, as for example, lifting height control are incorporated in the control unit of the control device for loading and unloading mechanism. Prior to the lifting work, an operator effects various settings according to the lifting height condition required for loading and unloading work and then starts a lifting height operation. In this instance, an automatic control system is constituted, which includes therein a valve opening control system provided with respect to a hydraulic pressure circuit for actuating a lift cylinder. The lifting height control is effected so as to control the valve opening control system due to the deviation between an actual lifting height above said setting value. However, when the setting is changed to a great extent according to the change of the loading and unloading working place, it is required to adjust the automatic control system in order to stabilize the control system. Alternately, it happens that the desired control accuracy cannot be obtained. Further, such a lifting height control is effected in a series of sequential control for loading and unloading work with the lifting height control being related to various kinds of controls. Accordingly, it is desirable to supervise the whole system control in view of the simplicity of the circuit and harmonious execution of the control.
In view of this, another attempt has been made. The programmed series of sequential control matching with the objective loading and unloading operation is stored in a computer, such as a microcomputer. When, for instance, lifting height control is effected, the concerned programmed routine for lifting height control is called from the program to effect a lifting height control due to the execution of the programmed routine.
In this instance, prior to lifting height work, the setting is effected by memorizing the objective lifting height into the microcomputer. When a push-button for starting an automatic lifting height is pushed, the execution of the program for lifting height control routine starts. Thus, the automatic control system including therein the above-mentioned valve opening control system becomes operative on the basis of the command being fed from the microcomputer so that the fork moves to the objective lifting height to automatically stop thereat. Accordingly, when the change of the setting is required, the changed lifting height is memorized into the microcomputer. When calling routines for lifting height control, it is sufficient to call the concerned routine in such a manner to distinguish it from the other.
These computer controlled devices for loading and unloading mechanisms are provided with a pair of limit switches for setting a horizontal position of a fork and for setting an angle or running attitude of the fork responsive to the tilt cylinder for tilting the upright in the forward and backward directions along which the fork is slidably provided. When a horizontally positioning push button switch is pushed in order to horizontally position the fork at the lifting height position in the working place, the fork is moved from an inclined position to the holizontal position and is stopped thereat. When a push button switch for taking the fork to the running attitude position is pushed, the fork is moved to the predetermined position suitable for running and at the same time is rotated to the predetermined inclined position suitable for running, and is stopped thereat.
However, when the limit switch for setting an angle for running attitude becomes operative, the fork is always stopped at the predetermined inclined position. Accordingly, it is impossible to adjust the fork so that the angle of the fork is suitable for different kinds and shapes of loads. For this reason, it is likely that the load will be damaged or an unstable running condition will be provided.
In general, as the lifting height of the loaded fork increases, the attitude thereof becomes unstable. However, it is solely the horizontal position of the fork and the running attitude thereof which are controlled. As a result, it is difficult to adjust a backwardly inclined angle of the upright suitable for lifting height of the fork. If the backwardly inclined angle of the upright is set to be large, when the fork is lifted to the height lifting position, the center of gravity of the upright becomes unstable, which brings about a safety problem.